1. Field of the Invention
This invention relates to non-woven mats of randomly oriented metal fibers, to metal/polymer composites formed with such mats, and particularly to thermoformable metal/polymer composites.
2. Description of the Related Art
A variety of composites containing both metal and polymeric materials are known for use in many varied applications. One important use for a metal/polymer composite is as a shield for electromagnetic and radio frequency waves. The interference caused by such waves in electronic devices is commonly referred to as electromagnetic interference (EMI) or radio frequency interference (RFI), hereinafter jointly referred to as EMI. EMI shielding is often placed around an EMI source to prevent it from radiating EMI and interfering with surrounding devices. Also, the devices themselves may be provided with EMI shielding in an effort to shield the device from incoming electromagnetic radiation.
Another important use for such composites is for the protection of sensitive electronic parts from static charges. Static charge build-up can result from, e.g., the tribo-electrification of surfaces, and can lead to high electrical potentials. A sensitive electronic part that may come into proximity or contact with a statically charged surface can be damaged or destroyed. During shipping or handling, shielding of an electronic part from static electricity can be accomplished by placing the part in an electrically conductive metal or metal/polymer container, with the metal providing a shielding or Faraday cage-like surface to protect against static fields. Many applications require that the shielding be thermoformed into a particular shape or structure. A thermoforming process comprises heating the material and forming it into the desired shape. For example, thermoforming is used to make various types of containers or housings. Conventional thermoforming is usually accomplished by heating a thermoplastic sheet above its softening point, and forcing it against a mold by applying vacuum, air pressure and/or mechanical pressure. When the sheet is cooled, the contour of the mold is reproduced in detail.
It is possible to provide thermoformable articles with properties of EMI shielding. The methods used are, however, time consuming and relatively expensive. One reason for this is that metal cladding generally occurs after the shape or structure has been thermoformed, requiring a separate, secondary operation. Environmental concerns also exist due to some processing byproducts. Also, such coatings tend to chip or peel off after aging, thereby limiting their use as EMI shields.
Metal/polymer composites having continuous coatings of metal deposited by sputtering, arc spraying, vapor deposition or the like are known in the art.
U.S. Pat. No. 3,272,292, (Nicely), discloses a non-woven unitary metallic sheet which is fabricated by extruding a molten stream from a metallic melt into an atmosphere which reacts to form a stabilizing film about the periphery of the metal stream. The spun metal filaments are allowed to solidify, and then collected as a nonwoven fibrous mass. The mass of filaments are then compressed into a sheet-like form, and given strength by binding all or selected adjacent fibers together.
U.S. Pat. No. 3,565,127, (Nicely et al.) discloses a composite yarn structure comprising strands of brittle filamentary materials collimated into a bundle and having a wrapped or braided covering applied to hold the bundle as a unit until further processing. The filamentary materials may be selected from inorganic refractory fibers such as carbon, boron, quartz, silicon carbide, and the like. Filling yarns for weaving the bundle may be made from a large variety of high-modulus, high-strength materials.
An alternative to continuous metal coatings is a high loading of short, straight, staple metal fibers either mixed within the thermoplastic sheet material or coated across its surface. The effectiveness of the EMI shielding is related to the overall electrical conductivity of the metal/polymer composite, with higher electrical conductivity typically giving better EMI shielding. When electrical conductivity is obtained by mixing conductive fibers into the volume of a polymer or forming a coating of those fibers on the surface of the polymer, the overall conductivity is dependent on the individual conductivity of the fibers as well as the number of electrical contacts between said fibers. Short staple conductive fibers require higher concentrations in order to obtain sufficient electrical contact between the fibers. High loading of metal fibers tends to adversely affect the physical properties of the final metal/polymer composite. With any conductive fiber, the number of contacts is significantly reduced, as the polymer containing them is stretched, during the thermoforming process, which causes a concurrent reduction in the EMI shielding.
U.S. Pat. No. 4,678,699, (Kritchevsky et al.), discloses a stampable thermoplastic composite having at least one thermoplastic layer and at least one shielding layer, the composite having an EMI/RFI shielding effectiveness of at least 30 dB. In one embodiment the shielding layer may be a knitted wire mesh or screen. The shielding layer may be adjacent to the thermoplastic layer, or it may be embedded into the thermoplastic layer.
Another alternative is the use of metal coated polymeric fibers in place of solid metal fibers in filled or coated plastics for EMI shielding. These composites provide reduced weight and some cost savings over their solid metal fiber filled counterparts. However, they have significantly reduced EMI shielding properties when compared to solid metal fibers. This reduction is due to the electrically conductive surface of the fibers being typically destroyed as they are deformed or stretched with the thermoformed substrate.
Another alternative to continuous metal coatings applied after the thermoforming operation is the use of blanks (section of polymeric substrate purchased by the thermoformer from his extruder supplier) that have been prepared from various metal laminates. The metal layers in such preassembled blanks consist of anything from thin foils to fibrous coatings. These materials are usually incapable of withstanding the severe dimensional changes, i.e., up to 300% elongation, during the thermoforming process. Usefulness of such materials in EMI shielding applications requiring a thermoformed article are therefore highly dependent on the ductility of the metal used. See, e.g., U.S. Pat. No. 4,689,098 (Gaughan).
JAP 1990-276297 discloses molded EMI shielding sheets prepared by arranging narrow long metal fibers in a mat, adhering synthetic resinous films over top and bottom of the mat, and then vacuum forming. The long metal fibers are disclosed to reduce the chance of puncture of the synthetic resin films. Between the film layer and the metal mat, a nonwoven cloth made from synthetic resins may be layered. The nonwoven web is then entangled with the metal fibers by needle punching both mutually, and film layers are then sandwiched on both sides of the cloth, and the article is vacuum formed.
JP62-176823, published Aug. 3, 1987 discloses compression molding of a thermoforming laminated sheet consisting of alloys with high plasticity and thermoforming resins wherein a lower pressure is applied initially and the pressure is gradually increased as the molding continues. This process is disclosed to reduce cracking and peeling of the final thermoformed article.
Several alternative solutions have been attempted to improve the effectiveness of conductive fiber based EMI shielding. Forming pressure welds or sintered bonds between the fibers improves electrical conductivity, but reduces overall flexibility and extensibility of the welded mat. A sintered or otherwise bonded metal-fiber/polymer composite sheet cannot be thermoformed except at stresses which would break the fibers themselves, the bonds between the fibers or both, thus drastically reducing the shielding properties.
Electrical conductivity could be enhanced in conductive fiber filled or coated composites by using longer fibers that would require fewer fiber-fiber contacts to maintain continuous electrical conductivity. However, since metal fibers in their solid state stretch little, if at all, under thermoforming conditions, the fibers must slip relative to the polymer to keep from breaking during the thermoforming process. The fiber changes its shape during the deformation of the polymer matrix. The net effect is a dramatic increase in the thermoforming stress required for plastic flow of the composite which again would break the fibers themselves, the bonds between the fibers or both. Also, at concave surfaces or corners, the fibers may pull free of the surface.
Additionally, baffles are often used within the heating chamber of the thermoforming machine to preferentially heat certain areas of the substrate polymer to be thermoformed to aid in wall thickness control of the final article. Any metallic layer contained within the sheet will disturb the heating characteristics of the sheet during the heating cycle. Such reflective and absorptive interference with the desired heating pattern are unavoidable when using EMI shielding laminates and compositions of the prior art.
Contrary to what might be expected, the amount of EMI that passes through a given void is dependent on the length of the void's longest dimension, and not on the total area of the void. For illustration, a 1.00 mm by 1.00 mm void (area=1.00 mm.sup.2) is believed to let less EMI pass through than would a 3.0 mm by 0.05 mm void (area=0.15 mm.sup.2) even though the 1.00 mm.sup.2 space has greater than six times the area of the 0.15 mm.sup.2 space. This is sometimes called "the slot effect".
Accordingly, even extremely thin openings must be avoided where the opening has a substantial longitudinal dimension. A multi-part EMI shielded enclosure can be effectively sealed without leaving thin cracks between adjacent enclosure parts by providing conductive surfaces on adjacent surfaces of the shielded enclosure. For this reason it is not always highly effective to simply extrude or laminate fibers, particles, or flakes of metal into bulk polymer. The metal must preferably be distributed throughout the polymer to specifically include the sealing surfaces or at least be accessible for electrical connection to the opposing surfaces by way of special gaskets or electrical connectors.
Additionally, sheets of the prior art made for the purpose of providing EMI shielding as removed directly from the thermoforming machine (e.g. one step shielding) require that the entire thermoformable blank contain the metal laminate or shielding layer throughout. This results in excess cost for unused shielding material since these blanks are typically sold in large rectangular sections. Also, the resulting edge trim (the portion of the material not included in the final thermoformed article) from such material cannot be ground up and reused since it is contaminated with the metallic filler.
Accordingly, there is a need for an add-on EMI or static shielding sheet material which can be placed onto a thermoformable polymeric substrate solely in those areas which are to be shielded. Additionally, there is a need for a material which will reduce cost and increase processability, allowing the thermoformer to conveniently and cost effectively provide shielding for specific thermoformable articles in large scale production as well as in limited prototype configurations.
Such a thermoformable add-on EMI shielding sheet also must not entrap significant amounts of air between itself and the substrate polymer material in order to reduce the propensity of the conductive add-on coating to break free from the polymeric substrate during removal from the thermoforming equipment or during subsequent handling and use of the thermoformed article. In addition, such pockets of air will tend to expand under the heat of the thermoforming operation and burst causing large holes to be formed in the EMI shield severely compromising its effectiveness.
It has now been discovered that a polymeric carrier material which is either porous or capable of becoming porous during the thermoforming process, and having a metal mat at least partially embedded therein comprising a plurality of fine, randomly-oriented metal fibers, wherein the carrier material softens and the metal fiber mat has a melting temperature lower than the maximum thermoforming temperature provides such an add-on sheet.
Preferred add-on sheets function without requiring masking of cosmetic surface areas of the thermoformed article, a separate process which is required prior to applying conductive paint. Also eliminated is the necessity for repainting decorative areas of the thermoformed article such as is required when arc spraying or plating procedures are used.
Preferred add-on EMI shielding materials provided herein do not affect the preferential heating characteristics of the polymeric substrate blank and will not increase significantly the cycle time for thermoforming. The material also reduces waste by allowing edge trim to be reground for reuse by the extruder.